Electrostatic precipitators are an efficient and economic way of collecting particulates suspended in a waste gas stream. The electrostatic precipitator technology was first invented and implemented in the early 1900s by Research-Cottrell, the predecessor in interest of the assignee of this application.
In the electrical precipitation process of an electrostatic precipitator, a chamber filled with large parallel spaced conductive panels, referred to as collecting electrodes, are supported in parallel relation from anvil beams. In a typical application, the collecting electrodes are rigid and may be about 10 to 55 feet in length, have a width of between about 4 and 12 feet and weigh between 400 and 2,000 pounds or greater. Further, an electrostatic precipitator may include about 10 to 400 collecting electrodes. In a preferred embodiment of a collecting electrode developed by the predecessor in interest of the assignee of this application, the collecting electrodes each include vertically extending planar collecting portions separated by vertical tubular portions. In a preferred embodiment, the tubular portions are generally diamond-shaped, preferably having rounded edges and thus the collecting electrodes include a triangular shaped projecting portion on opposed sides of the collecting electrodes as will be understood by those skilled in this art. The collecting electrodes may be made by forming two generally planar metal panels, forming parallel triangular-shaped projections and welding the panels together to form planar collecting portions separated by diamond-shaped tubular portions.
At the center point, running parallel between the collecting electrodes, are a series of individual discharge electrodes that run vertically the full height of the collecting electrodes. These may be small diameter wires or more typically today rigid “mast-type” assemblies having pointed projections. The discharge electrode assembly is supported on an insulated assembly to keep the discharge electrodes electrically separate from the collecting electrodes.
A high voltage direct current (D.C.) is applied to the opposing surfaces of the collecting and discharge electrodes, wherein the positive charge (+) is applied to the collecting electrodes and the negative charge (−) is applied to the discharge electrodes. With electron flow from negative to positive, the small surfaces of the discharge electrodes emit a field of negative electrons or ions in the space between the collecting electrodes. When a particulate laden or polluted waste gas is passed at low velocity through this electron field, the particulates in the gas stream will become negatively charged. The negatively charged particles will then be attracted to the positive charge on the collecting electrodes. When this migration toward the surface is complete, the inherent resistivity of the particles will prevent complete loss of the charge through the collecting electrode surface. The retained opposing electrical charge in the particles will cause the particles to agglomerate or stick to the surfaces of the collecting electrodes. Electrostatic precipitators have now become the equipment of choice in pollution abatement applications, wherein the gas stream has fine particulate material in the exhaust gas, including industrial and utility coal and oil fired boilers, the paper and pulp industry, refineries and other pollution abatement applications. In the last half of the twentieth century, as such industries grew and environmental issues became more important, there was a big demand for larger and more efficient electrostatic precipitators. More recently, environmental regulations have become so strict that even the slightest emission violation or a fundamental loss of a part of a precipitator, can result in heavy fines and production cut-backs and shut down.
These requirements have caused major changes in the physical design of electrostatic precipitators, including greater sectionalization of the electrostatic precipitators having several small electrical sections or chambers to increase efficiency and reduce loss percentage in the event of a failure and changes in the design of many of the system components. Two of the main changes have been in the area of collecting and discharge electrodes. While the original small diameter wire design was very efficient electrically and cost efficient, the small diameter wire design was prone to breakage and failure, particularly due to age, sparking and stress from the precipitator internal cleaning rapping or vibration system which causes the agglomerated particulates to fall from the collecting electrodes. Wire discharge electrodes are being replaced with rigid mast-type electrodes, which are more rugged in design. Collecting electrodes also had to be made stronger so that they could maintain closer tolerances and surface design had to be improved to make them more efficient in both material collection and cleaning.
As will be understood by those skilled in this art, the positively charged collecting electrodes collect particulate materials which must be periodically removed from the collecting surfaces. The particulate material is removed from the collecting electrodes by “rapping” forces applied to the collecting electrodes. Rapping forces may be applied to the collecting electrodes by vibrators, hammers or magnetically, and the dislodged particulate material then drops into collecting hoppers located below the collecting electrodes. Thus, the collecting electrodes must be able to withstand and provide uniform rapping forces throughout the plate surfaces for overall cleaning efficiency. The Opzel™ collecting electrode available from the assignee of this application includes vertical planar collecting surfaces separated by vertical diamond-shaped tubular portions having triangular projecting surfaces on opposed sides of the collecting electrodes, as described above, together with improved rigid mass-type discharge electrodes which has proven to be a reliable answer to the problems of discharge and collecting electrode failure. However, while the issues of normal operational collecting and discharge electrode failure has been resolved, there will still be failures that relate to general aging, or failure due to temperature surges caused by process upset conditions or precipitator fires which can damage or destroy the internal components of the electrostatic precipitator.
As will be understood by those skilled in this art, it is very difficult and expensive to replace the collecting electrodes of an electrostatic precipitator. Replacement of the collecting electrodes results in lengthy down time for the precipitator, always requiring that the entire electrostatic precipitator and process be shut down. To replace conventional rigid collecting electrodes, it is generally necessary to cut holes in the precipitator roof, also generally requiring cutting holes in surrounding building structure and cranes to lift and lower the collecting electrodes into place. There are also many instances where the owner of the electrostatic precipitator desires to upgrade an older existing precipitator that has good external casing, but may suffer from frequent failure of the internal electrical components or require efficiency or reliability upgrades. In those cases, it is necessary to remove the upper structure of the precipitator and employ cranes and large forces of welders and laborers to perform the upgrade. Replacement of the collecting electrodes also requires shut down of the apparatus generating the waste gas stream.
Thus, there has been a long felt need for a method of replacing collecting electrodes of an electrostatic precipitator which substantially reduces extensive down time for the precipitator, avoids cutting large holes in the precipitator roof and the surrounding casing and large cranes to lift and lower the collecting electrodes in place and eliminates the requirement for special tools and welding equipment. The method of making replacement collecting electrodes for an electrostatic precipitator of this invention solves these problems by forming or making small collecting electrode sections which may easily be shipped from the manufacturing site to the precipitator and passed through a small opening in the precipitator casing and reliably reassembled in the cramped conditions within a precipitator. Further, the collecting electrode sections of this invention may be reassembled into a rigid large collecting electrode able to withstand and transmit rapping forces for cleaning and has all of the advantages of a conventional modern rigid collecting electrode. Other advantages and meritorious features of this invention will be more fully understood from the following summary of the invention, description of the preferred embodiments and the appended drawings.